The use of permanent magnet motors in severe-duty applications is often hampered by difficulties in protecting the rotor magnets and retaining them on the rotor rim during high centrifugal loading. Under severe cyclic duty, rare earth magnets, e.g., NdFeB magnets, which can be very brittle, can experience heavy shock and vibration loads as well as high centrifugal forces. In some cases, the rotor experiences frequent and abrupt torque reversals that demand that the retention system also minimize assembly tolerances. Excessive magnet/retention system assembly clearances could permit movement of the magnets within the retaining configuration and cause damage to the magnets themselves. To compound matters, the magnets are inherently highly susceptible to chemical, corrosion and abrasive corrosion environmental attack and require a durable and resilient means of protection.
For larger permanent magnet motors, most magnet and pole retention systems utilize either circumferential composite outer bands or radial retention fasteners. In both cases, the retention forces are applied radially inward onto the magnets and directed toward the rotor axis or centerline to tighten the assembly and thereby more securely fasten the pole pieces to the rotor. Ideally, the retention systems should evenly distribute the radial forces on the poles such that the pole pieces are retained uniformly at all points along the axial length of the pole pieces to prevent uneven support during centrifugal loading. In practice the ideal is difficult to reach.
In nearly all cases, the retention systems are not ferromagnetic (that is to say, these systems, or at least those portions used to secure the poles, have relatively low magnetic permeability) in an effort to limit magnet flux leakage via the retention systems. Such leaked magnetic flux diminishes torque production. However, as a result, these systems increase the effective magnetic air gap, which in turn increases magnetic circuit reluctance. The increased reluctance weakens the flux emitted by the magnet within the circuit and increases its leakage flux. These both in turn also diminish torque production.
There is a demand of late for motors having lower noise characteristics. To satisfy this demand, as well as a desire for sinusoidal back EMF (electromotive force), rotor poles are shaped. This is called pole shaping. In some magnet retention systems, such as those utilizing hoop/band retention devices, it is difficult to use shaped poles because shaped poles can wear non-uniformly at their outer diameter.
Various systems and configurations for securing pole pieces have been described in the patent literature.
U.S. Pat. No. 6,548,932 to Weiglhofer, et al, describes U-shaped channel pieces oriented radially outward with permanent magnets contained within the channels defined by the U-shapes. Non-magnetic channel pieces are positioned between wedge-shaped ferromagnetic pole pieces. The channel pieces utilize a T-shaped protrusion at the base of the pieces to positively engage in a similarly shaped slot in a non-magnetic rotor surface to retain the poles pieces in the channels. No fasteners are required.
U.S. Pat. No. 5,973,435 to Irie, et al, describes a motor in which a series of soft resinous and non-magnetic magnet carrier members and cover faces are used. The magnets are secured to a non-magnetic retaining band or belt by means of the carrier members and the resulting band and magnets structure is secured about the rotor by means of the band.
U.S. Pat. No. 5,811,908 to Iwata, et al, describes a U-shaped retention channel constructed of ferromagnetic material. The channel is positioned over a permanent magnet and between adjacent pole pieces. The ends of the retention channel include lateral protrusions that interlock under corresponding protrusions extending from the adjacent pole pieces. The pole pieces are contoured to the shaped of the rotor outer surface and permanent magnets.
U.S. Pat. No. 4,179,634 to Burson describes a system in which a permanent magnet is restrained radially by means of ferromagnetic poles pieces located on either side of the magnet. The pole pieces interlock with protrusions in the rotor at the base of the magnet cavity and above the magnet itself by means of locking tabs located above the magnet. The entire assembly is secured by means of tapered roll pins made of ferromagnetic spring steel that extend the length of the magnet. The tapered roll pins draw the magnet radially inward within the cavity when inserted.
U.S. Pat. No. 4,745,319 to Tomite, et al, describes a U-shaped channel configuration used to secure magnets in a stator yoke assembly. The channels are oriented with the tops secured to the stator at the open ends and the closed ends placed within elastic retainers situated in grooves within edges of the permanent magnets. The channels are constructed of elastic non-magnetic material.